Modified precipitation processes and optimized copper content of CuO–CeO2 catalysts for water–gas shift reaction

The precipitation processes of CuO–CeO2 catalysts preparation were modified, and their optimized copper content were also studied in detail. As indicated by the experimental results, the WGS catalytic activities of the CuO–CeO2 catalysts can be ranked as: stepwise precipitation (SP) > deposition–precipitation (DP) > co-precipitation (CP), suggesting that stepwise precipitation (i.e., a modified DP) is a convenient and effective method to prepare CuO–CeO2 WGS catalysts. The optimized copper contents of CuO–CeO2–SP and CuO–CeO2–CP catalysts are 20 wt.% and 25 wt.%, respectively. Their catalytic activities can be strongly correlated with the results from XRD, XPS, Raman, N2-physisorption, N2O chemisorption and H2-TPR. For DP and SP, a certain amount of copper has substitutively incorporated into ceria lattice with multiple Ce3+ and oxygen vacancies, which is considered as one relatively strong interaction between copper and ceria support. The substitutional incorporation of copper creates larger lattice distortion, lattice defect (embodying as larger lattice cell contraction of CeO2, microstrain and Raman shift) and stronger reducibility (i.e., lower reduction temperature of H2-TPR), as a consequence, higher surface energy and catalytic activity for WGS reaction. While for CP, copper nitrate and cerium nitrate were simultaneously precipitated, resulting in the burial of many copper species in ceria supports, i.e., occupancy incorporation of isolated copper into ceria lattice vacant site, which is considered as another weak interaction between copper and ceria support. Accordingly, combined with N2O chemisorption results, it is demonstrated that surface copper species of CuO–CeO2–CP are fewer and less active than those of CuO–CeO2–DP and CuO–CeO2–SP. Lastly, there is a direct relationship between the pore volume along with most probable pore size of the as-synthesized catalysts and their corresponding catalytic activities for WGS reaction.